The present invention relates to a strap for diving face mask and more particularly to such a strap having a specific configuration as well as a high functionality in a region thereof adapted to come in contact with the occipital region of the wearer's head.
Conventional diving face masks, for example, one of them as disclosed in Japanese Patent Publication No. 3536004 includes a strap to be put about the wearer's head and such strap is provided in its lengthwise middle zone with a loop section adapted to be held in contact with the occipital region of the wearer's head. The loop section comprises an upper branched sub-section and a lower branched sub-section which are symmetric to each other in a vertical direction as well as in a transverse direction.
Such a conventional strap will be described in more detail with reference to FIG. 8 of the accompanying drawings as showing prior art. The strap 100 includes in its lengthwise middle zone a loop section 101. The loop section 101 comprises an upper branched sub-section 102 and a lower branched sub-section 103 which are symmetric to each other about a first central axis y extending in the transverse direction as well as about a second central axis x extending the vertical direction. In other words, the circumferential length of the upper branched sub-section 102 is exactly the same as the circumferential length of the lower branched sub-section 103. The strap 100 linearly extends along the first central axis y over the entire length. Though not shown, the loop section 101 is two-dimensionally flat as viewed in a sectional view of the strap 100 taken in the vertical direction.
In the case of the conventional strap 100 for diving face mask as has been described above, the upper and lower branched sub-sections 102, 103 constituting together the loop section 101 have exactly the same in the circumferential length and this feature causes a problem as will be described below.
In general, the occipital region of human head (the wearer's head) is three-dimensionally curved so as to have a convexity directed backward and a crowning of such convexity is positioned virtually on the horizontal plane passing the wearer's eyes or slightly above this horizontal plane. Assumed now that the wearer's head is divided into upper and lower sections about the crowning, the circumferential length of the lower section is shorter than the circumferential length of the upper section. With the strap 100 put on the head, the upper branched sub-section 102 is positioned on the upper section of the head and the lower branched sub-section 103 is positioned on the lower section of the head. Consequentially, the lower branched sub-section 103 necessarily becomes slack and/or redundant and a good fit to the lower section of the head is not obtained. To overcome such inconvenience, the wearer usually adjusts the fit, for example, by pulling down the lateral sections of the strap 100. However, such fit adjustment may be inadequately effective or effective temporarily.